It is well known in the art that some types of crops are degraded by the UV-components of solar radiation, which must be filtered off to obtain high quality and productivity of the crops. Additionally, some microorganisms, e.g. Botrytis Cinerea, as well as some harmful insects, e.g. white flies, aphides, thrips or leafminers, can proliferate under specific UV-irradiation. These pests can be significantly reduced when UV light does not or to less extent reach the plants. [R. Reuveni et al., Development of photoselective PE films for control of foliar pathogens in greenhouse-grown crops, Plasticulture No. 102, p. 7 (1994); Y. Antignus et al., The use of UV absorbing plastic sheets to protect crops against insects and spread of virus diseases, CIPA Congress March 1997, pp. 23-33]. On the other hand, bee activity, requiring a certain band of UV radiation, needs to be retained in greenhouses in order to ensure fructification on flowering plants, e.g. tomato, cucumber, pumpkin etc.
Changes in the solar radiation effected by aptly placed materials can impact plant growth indirectly by changing other environmental factors. The more directly impacted environmental factor is temperature. Indeed, one of the principal original aims of greenhouses was to increase the temperature gain during the day and to decrease the temperature loss at night relative to the unprotected environment. Such benefit is being improved upon and fine-tuned by using materials that avoid excessive daytime heat gain or further decrease nighttime heat loss by regulating radiation flux in the near infrared (NIR, 700-2000 nm) and mid infrared (2-20 μm) regions of the electromagnetic spectrum. To this category, for example, belong materials claimed in WO-A-9405727, U.S. Pat. No. 4,895,904 and EP1652422. The present invention does not impact these frequency ranges.
Plant growth is more directly regulated by photosynthesis, photomorphogenesis and photoperiodicity. All these processes require light and contribute in a unique way to plant development. If the spectrum of the outside solar radiation can be significantly modified by the optical properties of the glazing or film covering a greenhouse or of a mulch covering the ground, a change in plant growth may occur. Changes in the radiation transmitted or reflected by the agricultural film induce photosynthetic and photomorphogenic effects and can result in modifications of the metabolism, architecture and shape of the plants with significant consequence on the value of the crop.
Light used by the plants for their energy needs is that falling within the PAR region (Photosynthetically Active Radiation), defined as all photons between 400 and 700 nm. The best known photomorphogenic parameter used by plants is the ratio of Red (600-700 nm) to Far Red (700-800 nm), as for example disclosed in EP1413599. On the other hand, the impact of higher energy blue light and very near UV, 380-500 nm, is also very important, as shown for example in Brian Thomas and H. G. Dickinson, Evidence for two photoreceptors controlling growth in de-etiolated seedlings, Planta No. 146 p. 545-550 (1979), although much less well understood. This wavelength range is the region of primary interest in the present invention
Many attempts have been made to influence plant growth by manipulating natural light passing through coverings such as those employed in greenhouses and tunnels, or reflected by mulching sheets, starting at very early times [see, for example, Delaroquette, M., Biologic action of sunlight, the Journal of the American Medical Association No. 66, p. 65 (1915)]. One straightforward strategy is the selective absorption of specific light wavelength ranges. For example, increase in Red/Far Red can be obtained by using systems in which light of wavelength around 730 nm is absorbed preferentially relative to light of wavelength around 660 nm: these wavelengths are the maxima in the absorption spectra of the photo-interconvertible isomers of the phytochrome photoreceptor. Several patents claim such an effect, such as GB2314844, EP1080878 and U.S. Pat. No. 6,441,059. There are also some commercial products making the obtained Red/Far red increase a claim in their marketing literature (the “Solatrol” film by British Polythene is one such example, targeted at making more compact decorative plants).
However, the lack of knowledge about the reaction of plants to modifications in the intensity of other specific light wavelength ranges has made it until now impossible to make truly effective products. Indeed, many times it has been stated that an advantage had been found by means of a specific color or, more often, any color, as in WO-A-9405727, US3542710, EP1582555 and RO116242. The colors are variously defined as those of the material or those of the transmitted or reflected radiation. More often than not, the colors are not spectrally defined beyond their culturally-determined names (yellow, green, red, blue and so on). Sometimes they are defined by specifying the pigments or dye employed to obtain them, but often, such as in RO116242, the commercial names provided are too generic and do not define univocally a compound or a spectrum, making it impossible to replicate or exploit the findings. Similarly, WO-A-9405727 claims any colored material obtained by adding inorganic interference pigments to polymer films, especially those pigments preferentially reflecting or transmitting green light. However, none of these materials have found use in practice, due mostly to the poor spectral selectivity of the physical effect on which they rely, namely two-layer interference.
Indeed, the research leading to the present invention has shown that plants “see” colors very differently than humans do, and that they are sensitive to spectral variations both more subtle in intensity and more defined in wavelength than what is apparent to the human eye.
Surprisingly it has been found in comparative experiments, both under controlled laboratory conditions and under real-life conditions, that there is a positive reaction of plants to a specific solar spectrum modification in the 380 to 500 nm range, whereas other spectral modifications of the same wavelength region resulting in colors very similar for the human eye had either no effect or a negative impact on plants. The spectral modification that is the object of the present invention cannot be described under the simple color ratios (Red/Far Red, Blue/Red and so on, as in WO-A-9405727) usually employed in both the patent and the scientific literature to describe the reaction of plants to light quality modifications.